In a last-minute effort to sink the Republican health care bill, a powerful network of conservative donors said Wednesday it would create a new fund for Republican 2018 reelection races — but they’ll only open it up to GOPers who vote against the bill.

The advocacy groups helmed by Charles and David Koch have unveiled a new pool of money for advertisements, field programs and mailings that would exclude those who vote for the health care bill they oppose on Thursday. The effort, which they described as worth millions of dollars, is an explicit warning to on-the-fence Republicans from one of the most influential players in electoral politics not to cross them.

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The Koch-aligned networks oppose the bill because they think it does not do enough to scale back former President Barack Obama’s health care policies.